The Fenix LD12, 2017 edition is constant current for both AA and 14500 batteries. I think I know where you are going with this question. The forward voltage drop of an LED is around 3.0v. That means if using an AA battery, the driver has to be a boost circuit. But if running a 14500 lithium ion which can have a voltage up to 4.20 volts, then the driver has to have a buck (step down) type of circuit. So the challenge to the manufacturer is to have a cost effective efficient driver that can handle all voltages from AA and 14500. Some single AA lights that can accommodate the 14500, elect to have a good efficient constant current boost type of driver for AA but when using a 14500, the driver switches to PWM. I've seen that with the Nitecore MT10A and I think the Jetbeam Jet I MK is that way too. There are probably others as that is probably the easiest way to deal with this range of voltage, without too expensive of a buck-boost driver of some type.
On a side note, if wanting to run the 14500 rather than an AA, I have found the 16340 lights to be slightly more efficient than the 14500 lights...........at least on a couple of samples that I had such as the Fenix PD25 and E15 vs the Fenix LD11 and LD12-2017 on a 14500. Since these are all lights by the same manufacturer, it tells me that it must be a little more challenging to make a driver for the AA/14500 vs the CR123A/16340. I posted that information elsewhere on CPF but for some reason I got better performance (output x time) out of the 16340 powered lights than the 14500 powered lights considering the 16340 that I used was 100mAh less.
So would I dump my AA/14500 lights in favor of the 16340 only? No. There is some advantage to having a light that can run AA batteries, because it makes it more versatile in a crises situation where you may be scrambling to find batteries. Meanwhile, I have a 14500 in both the LD11 and LD12-2017 knowing full well that I can substitute an AA if needed.